The present invention relates to a cathode electroactive material for use in lithium ion secondary cells, a process for producing the material, and a lithium ion secondary cell using the cathode electroactive material.
Lithium manganese composite oxides (hereinafter referred to as Lixe2x80x94Mn composite oxides), which are very safe and are produced from abundant natural resources, have been of interest for use as a cathode electroactive material for lithium ion secondary cells. However, Lixe2x80x94Mn composite oxides exhibit poor discharge capacity per amount of an electroactive material as compared with lithium cobalt composite oxides (hereinafter referred to as Lixe2x80x94Co composite oxides). In addition, secondary particles of Lixe2x80x94Mn composite oxide are lightweight and absorb a large amount of oil, because the particles contain many pores. Thus, the amount of electroactive material which can be fed into a dimensionally limited cell must be restricted, thereby disadvantageously lowering the electrochemical capacity of a unit cell.
In recent years, U.S. Pat. No. 5,807,646 (Japanese Patent Application Laid-Open (kokai) No. 9-86933) has proposed measures to counter the aforementioned problem. Specifically, a mixture of a manganese compound and a lithium compound is shaped at a pressure of 500 kg/cm2 or higher, heated, and crushed, to thereby produce an Lixe2x80x94Mn composite oxide having a tapping density (i.e., apparent density of powder in a container which is moved, e.g., vibrated under certain conditions) of 1.7 g/ml or higher. However, the disclosed tapping density is at most 1.9 g/ml, which is unsatisfactory.
The above official gazette also discloses the average particle size of secondary particles which are formed by aggregating primary particles of an Lixe2x80x94Mn composite oxide. However, even when the packing density of secondary particles is enhanced through the interaction between primary particles, secondary particles are disintegrated during the electrode material (paste) preparation step. Thus, controlling the average particle size of the secondary particles is not a fundamental counter-measure.
Some methods for producing a spinel-type Lixe2x80x94Mn composite oxide have already been disclosed. Japanese Patent Application Laid-Open (kokai) No. 9-86933 discloses such a method comprising burning a mixture of a manganese compound and a lithium compound at a high temperature, e.g., 250xc2x0 C. to 850xc2x0 C. Japanese Patent Application Laid-Open (kokai) No. 4-237970 discloses such a method comprising mixing a manganese compound, a lithium compound, and an oxide of boron which can be substituted by manganese and burning the resultant mixture at a high temperature, to thereby produce an Lixe2x80x94Mnxe2x80x94B oxide in which Mn atoms are partially substituted with B, and the Lixe2x80x94Mnxe2x80x94B oxide serves as a cathode electroactive material.
When the aforementioned materials are burned at high temperature in air or in an oxygen gas flow, the secondary particles obtained through crushing have a high average porosity (15% or more) and a low tapping density (1.9 g/ml or less). Thus, thus-obtained cathode electroactive materials cannot be charged into an electrode in a large amount, and thereby, a high discharge capacity cannot be attained.
Japanese Patent Application Laid-Open (kokai ) No. 4-14752 discloses a cathode electroactive material employing a manganese oxide which is produced by mixing spinel-type lithium manganese oxide and titanium oxide and sintering the resultant mixture. However, disadvantageously, titanium oxide only reacts with lithium and manganese at 950xc2x0 C. to 1000xc2x0 C. or higher to form a melt, and a tapping density of 1.60 g/ml can be only attained by adding titanium oxide in an amount as large as 10 mass %.
An object of the present invention is to provide a cathode electroactive material for use in lithium ion secondary cells, which electroactive material has an excellent packing property and exhibits a high initial discharge capacity and a low decrease in discharge capacity after charging and discharging are repeated (hereinafter the property is referred to as high xe2x80x9ccapacity retentionxe2x80x9d).
The present inventors have conducted extensive studies, and have solved the aforementioned problems by successfully densifying particles of an Lixe2x80x94Mn composite oxide. Specifically, the spinel-type Lixe2x80x94Mn composite oxide is burned and crushed. Then, a sintering agent is added to the resultant pulverized particles, and the particles are granulated and burned.
Accordingly, the present invention provides a cathode electroactive material for use in lithium ion secondary cells, a process for producing the material, a paste for producing an electrode and a cathode electrode for use in lithium ion secondary cells comprising a cathode electroactive material, and a lithium ion secondary cell as described below.
[1] A cathode electroactive material for use in lithium ion secondary cells, wherein the cathode electroactive material predominantly comprises Lixe2x80x94Mn composite oxide particles with the spinel structure and particles of the electroactive material have an average porosity of 15% or less, the porosity being expressed by the following equation:
Porosity (%)=(A/B)xc3x97100xe2x80x83xe2x80x83(1)
(wherein A represents a total cross-section area of pores included in a cross-section of one secondary particle, and B represents the cross-section area of one secondary particle).
[2] A cathode electroactive material for use in lithium ion secondary cells as described in [1], wherein the average porosity is 10% or less and the average particle size of primary particles is 0.2 xcexcm-3 xcexcm.
[3] A cathode electroactive material for use in lithium ion secondary cells as described in [1], wherein the tapping density of the cathode electroactive material is 1.9 g/ml or more.
[4] A cathode electroactive material for use in lithium ion secondary cells as described in [3], wherein the tapping density of the cathode electroactive material is 2.2 g/ml or more.
[5] A cathode electroactive material for use in lithium ion secondary cells as described in [1], wherein the size of crystallites contained in the cathode electroactive material is 400 xc3x85-960 xc3x85.
[6] A cathode electroactive material for use in lithium ion secondary cells as described in [1], wherein the lattice constant determined with respect to the electroactive material is 8.240 xc3x85 or less.
[7] A cathode electroactive material for use in lithium ion secondary cells as described in [1], wherein the electroactive material is produced by granulating an Lixe2x80x94Mn composite oxide with the spinel structure serving as a predominant component comprising an oxide which is molten at 550xc2x0 C.-900xc2x0 C.: an element which forms the oxide: a compound comprising the element; an oxide which forms a solid solution or melts to react with lithium or manganese: an element which forms the oxide: or a compound comprising the element, and sintering the formed granules.
[8] A cathode electroactive material for use in lithium ion secondary cells as described in [7], wherein the oxide which is molten at 550xc2x0 C.-900xc2x0 C.: or the element which forms the oxide: or the compound comprising the element; or the oxide which forms a solid solution or melts to react with lithium or manganese: or the element which forms the oxide: the compound comprising the element, is at least one element selected from the group consisting of Bi, B, W, Mo, and Pb: or a compound comprising the element; a compound comprising B2O3 and LiF; or a compound comprising MnF2 and LiF.
[9] A process for producing a cathode electroactive material for use in lithium ion secondary cells predominantly comprising an Lixe2x80x94Mn composite oxide with the spinel structure, which comprises adding, to a pulverized Lixe2x80x94Mn composite oxide with the spinel structure, an oxide which is molten at 550xc2x0 C.-900xc2x0 C.: an element which forms the oxide: a compound comprising the element: an oxide which forms a solid solution or melts to react with lithium or manganese: an element which forms the oxide: or a compound comprising the element; and mixing, to thereby form granules.
[10] A process for producing a cathode electroactive material for use in lithium ion secondary cells as described in [9], which process comprises sintering the granules in addition to forming granules.
[11] A process for producing a cathode electroactive material for use in lithium ion secondary cells as described in [9], which process comprises, in addition to forming granules, sintering the granules by elevating the temperature of the granules from a sintering-shrinkage-initiating temperature to a temperature higher than the sintering-shrinkage-initiating temperature by at least 100xc2x0 C. at a rate of at least 100xc2x0 C./minute; successively maintaining the elevated temperature for one minute-10 minutes; and lowering the temperature to a sintering-initiating temperature at a rate of at least 100xc2x0 C./minute.
[12] A process for producing a cathode electroactive material for use in lithium ion secondary cells as described in [11], wherein the sintering is carried out by use of a rotary kiln.
[13] A process for producing a cathode electroactive material for use in lithium ion secondary cells as described in [10], wherein at least one element selected from the group comprising of Bi, B, W, Mo, and Pb: the compound comprising the element; a compound comprising B2O3 and LiF; or a compound comprising MnF2 and LiF is molten on the surfaces of particles of Lixe2x80x94Mn composite oxide so as to carry out the above described sintering process.
[14] A process for producing a cathode electroactive material for use in lithium ion secondary cells as described in [9], wherein pulverized Lixe2x80x94Mn composite oxide with the spinel structure has an average particle size of 5 xcexcm or less.
[15] A process for producing a cathode electroactive material for use in lithium ion secondary cells as described in [9], wherein pulverized Lixe2x80x94Mn composite oxide with the spinel structure has an average particle size of 3 xcexcm or less.
[16] A process for producing a cathode electroactive material for use in lithium ion secondary cells as described in [9], wherein granulation process is carried out through spray granulation, agitation granulation, compressive granulation, or fluidization granulation.
[17] A process for producing a cathode electroactive material for use in lithium ion secondary cells as described in [9], wherein at least one organic compound selected from the group consisting of acrylic resin, an isobutylene-maleic anhydride copolymer, poly(vinyl alcohol), poly(ethylene glycol), polyvinylpyrrolidene, hydroxypropyl cellulose, methyl cellulose, cornstarch, gelatin, and lignin is employed as a granulation aid during granulation process.
[18] A process for producing a cathode electroactive material for use in lithium ion secondary cells as described in [17], which process comprises binder removal process in air or in an oxygen-containing environment at 300xc2x0 C. to 550xc2x0 C.
[19] A cathode electroactive material for use in lithium ion secondary cells which is produced through a process as described in any one of [9] to [18].
[20] A paste for producing an electrode comprising a cathode electroactive material for use in lithium ion secondary cells as claimed in any one of claims [1] to [8].
[21] A cathode electrode for a lithium ion secondary cell, which the electrode comprises a cathode electroactive material for use in lithium ion secondary cells as described in any of [1] to [8] or [19].
[22] A lithium ion secondary cell equipped with a cathode electrode for a lithium ion secondary cell as described in [21].
[23] A lithium ion secondary cell as described in [22], which is formed into a coin-shaped cell, a coil cell, a cylinder-shaped cell, a box-shaped cell, or a lamination cell.